docs: def export(self, nidm_version, export_dir):
          """
          Create prov graph.
          """
          if self.expl_mean_sq_file is None:
              # Create Contrast Explained Mean Square Map as fstat<num>.nii.gz
              # multiplied by sigmasquareds.nii.gz and save it in export_dir
              fstat_img = nib.load(self.stat_file)
              fstat = fstat_img.get_data()
  
              sigma_sq_img = nib.load(self.sigma_sq_file)
              sigma_sq = sigma_sq_img.get_data()
  
              expl_mean_sq = nib.Nifti1Image(
                  fstat*sigma_sq, fstat_img.get_qform())
  
              self.filename = ("ContrastExplainedMeanSquareMap" +
                               self.num + ".nii.gz")
              self.expl_mean_sq_file = os.path.join(
                  export_dir, self.filename)
              nib.save(expl_mean_sq, self.expl_mean_sq_file)
  
          self.file = NIDMFile(self.id, self.expl_mean_sq_file,
                               filename=self.filename,
                               sha=self.sha, fmt=self.fmt)
  
          # Contrast Explained Mean Square Map entity
          path, filename = os.path.split(self.expl_mean_sq_file)
          self.add_attributes((
              (PROV['type'], self.type),
              (NIDM_IN_COORDINATE_SPACE, self.coord_space.id),
              (PROV['label'], self.label)))